1. Field of the Invention
The present invention relates to an improved apparatus for producing a sheet having many separate hermetically sealed hollow bodies from thermoplastic material such as low density polyethylene film.
2. State of the Art
Thermoplastic sheeting having many hollow bodies is used for such purposes as cushioning and insulation, under such names as "air cap" and "bubble pack".
In general, as shown in FIGS. 4A and 4B, this sheet is formed by heat fusing plastic film 2 (hereinafter referred to as "capfilm") having projections 2a, with flat plastic film 3 (hereinafter referred to as "backfilm"), to hermetically seal the open side of the capfilm and form cavity 4. There are some variations such as with an additional plastic film or other materials fused to the top of the projection of capfilm 2 for a three-layered product, or laminates with different materials such as paper.
The known art of producing the above-mentioned sheet having many hollow bodies is divided into the following three types. With the first type, plastic material is extruded from a die as a film and the capfilm is formed while in a plastic state. This capfilm comes into contact with a backfilm also extruded from a die, and the layers are fused by applying pressure. With the second type, plastic material is temporarily wound as rolled material, unwound and heated to a plastic state at a suitable temperature, and the above-mentioned forming and heat fusing are carried out. And with the third type, plastic material is extruded, heated to a plastic state immediately after rapid cooling, and the above-mentioned forming and heat fusing are carried out.
A disadvantage of the art of the first type is the loss of transparency as crystallization of the plastic material advances due to the backfilm being held at a high temperature for a long time. And compared to when the film is immediately cooled after being formed, there is another problem with low rigidity of the capfilm. Furthermore, creasing of the backfilm is easily created because it cannot be heat fused.
The offline production method of type 2 is inferior to the inline production method of type 1, from a cost point of view, but has the advantage of using simple facilities because it is possible to use rolled film that is easily procured. Among problems accompanying this technique are changes in the physical properties and surface condition of the film when crystallization of the plastic material and blooming of additives advances during storage. Because the physical properties of the plastic material are different depending on length of time and conditions under which it is stored, maintaining uniform quality of the product is difficult.
The method of type 3 appeared, in order to maintain the advantages of the inline method over the offline method and eliminate the above-mentioned weaknesses. Plastic material extruded from a die as a film is used after immediate rapid cooling and reheating, and along with being able to avoid opacity, a rigid, high-quality, uniform sheet with no creasing can be produced.
However, the method of type 3 is not necessarily easily put into practice. With conventional apparatus, temperature along the axis of the film heating roller and responsiveness to temperature are unsatisfactory, and maintenance is also troublesome.
Among heating means for the roller the easiest to use is an electric heater, but temperature uniformity along the axis of the roller is not good. Much time and trouble are required when the heater fails and has to be replaced. Although temperature rise is comparatively rapid, temperature fall must wait for heat radiation.
At factories with steam facilities, steam is also an effective heating means, raising temperature uniformity along the axis of the roller. But changes in the established temperature, which are dependent on regulation of steam pressure, are not rapid.
Circulation of a heat transfer medium is being used in place of steam to increase temperature responsiveness. Cooling the heat transfer medium at the outer portion of the roller can improve the rate of temperature fall, but temperature rise is not very rapid, and temperature uniformity along the axis is inferior to steam heating. With the intention of raising temperature uniformity along the axis, a jacket provided within the roller was conceived, enclosing a gas-liquid, two-phase mixture system, and having the advantage of being nearly maintenance-free. A coil is also provided within the roller, making possible a rapid temperature rise by heating through induction. However, temperature responsiveness is still insufficient because temperature fall requires time.
In addition, temperature distribution along the axis of the heating roller with a width of 1200 mm is generally .+-.5.degree. C., so that the temperature distribution along the width of the plastic film is not uniform, resulting in partial inferior forming or incomplete fusing. Particularly, the latter occurs easily. When using a certain kind of polyethylene as a representative plastic material for producing bubble pack, it is necessary to maintain the plastic film within a temperature range of 142.degree. C. to 144.degree. C. Temperatures of about 146.degree. C. and above cause adhesion to the heating roller. At temperatures of about 140.degree. C. and below, fusing is inferior. Accordingly, difficulties exist in producing, with a high yield, a wide range of products using an existing apparatus. This tendency is all the more evident when the intention is to obtain a lighter weight, flexible, low-priced product using a thin film.